This invention relates to apparatus for inspecting tubular members and more particularly relates to a system for inspecting hollow tube plugs installed in nuclear steam generators.
Before discussing the current state-of-the-art, it is beneficial first to briefly describe the structure and operation of a typical nuclear steam generator. In this regard, a typical nuclear steam generator comprises a vertically oriented shell and a plurality of U-shaped heat transfer tubes disposed in the shell. Pressurized radioactive primary fluid (e.g., water), which is heated by a nuclear reactor core, flows through the tubes as non-radioactive secondary fluid (i.e., water) of lower temperature circulates around the tubes. Heat is transferred from the primary fluid to the secondary fluid for producing steam in a manner well known in the art of nuclear-powered steam production. Such a nuclear steam generator is disclosed in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 to Robert A. Hickman et al.
It is important that the radioactive primary fluid remains separated from the non-radioactive secondary fluid so that the secondary fluid is not radioactively contaminated by the primary fluid. Contamination of the secondary fluid by the primary fluid may lead to excessive release of radioactivity to the environment. Excessive release of radioactivity to the environment is undesirable from a health and safety standpoint. Therefore, it is important that the tubes disposed in the steam generator remain leak-tight so that the radioactive primary fluid will not commingle with and contaminate the non-radioactive secondary fluid.
Occasionally, however, some of the tubes may experience degradation and may develop through-wall cracks due, for example, to stress corrosion cracking. These through-wall cracks may lead to commingling of the radioactive primary fluid with the non-radioactive secondary fluid. Therefore, the tubes are periodically inspected to identify those tubes suspected of being degraded or of having through-wall cracks. Heat transfer tubes suspected of being degraded or of having cracks are typically either sleeved or plugged, in a manner well known in the art, so that the primary fluid will not commingle with the secondary fluid. When a degraded tube is sleeved, a tubular sleeve is inserted into the tube and secured thereto, such as by welding, so that the degraded portion of the tube wall is covered. On the other hand, when a degraded tube is plugged, a tubularly-shaped hollow tube plug is inserted into one or both open ends of the tube so that radioactive primary fluid will not enter the degraded tube. Thus, in the case either of sleeving or plugging, the primary fluid is prevented from commingling with the secondary fluid even though the tube is degraded or cracked.
With respect to tube plugs, one type of tube plug suitable for use in nuclear steam generator heat transfer tubes is disclosed in U.S. Pat. No. 4,390,042 entitled "Tube Plug" issued June 28, 1983 to Harvey D. Kucherer et al. This type of tube plug comprises a shell having a closed top end and an open bottom end and having a tapered inner surface against which an externally tapered expander member moves by application of a pulling force. The expander member is drawn from the closed end to the open end of the shell. The motion of the expander member relative to the shell causes the shell to expand into contact With the heat transfer tube thereby plugging the tube.
However, the tube plug itself may become degraded and develop through-wall cracks. In this regard, it has been observed that the upper wall section of the tube plug is susceptible to stress corrosion cracking as a result of the substantial tensile stresses applied to the upper region of the walls of the tube plug by the expander member when it is drawn from the closed end to the open end of the tube plug. Such substantial stresses may eventually lead to through-wall cracks. A tube plug that is cracked may allow the primary fluid to enter the degraded tube in which the tube plug is disposed. If this occurs, there will be an increased risk that the radioactive primary fluid will commingle with the non-radioactive secondary fluid. In addition, if through-wall cracks develop in the upper region of the walls of the tube plug, there is a risk that the top of the tube plug will separate from the remainder of the tube plug and become a high-velocity projectile in the heat transfer tube. That is, the pressure of the primary fluid acting against the tube plug could under certain conditions separate and propel the degraded top portion of the tube plug upwardly within the heat transfer tube and damage the heat transfer tube, particularly in the U-bend region of the tube. Such severe damage to the tube may cause a breach of the tube wall. This is undesirable from a safety standpoint because such a breach could lead to commingling of the radioactive primary fluid with the non-radioactive secondary fluid. Consequently, it is desirable to inspect the upper wall section of the tube plug between the top of the tube plug and the top of the expander member to determine if this region is degraded or cracked.
However, the expander member presents an obstacle to inspecting the upper interior region of the tube plug located between the top of the tube plug and the expander member. Therefore, a problem in the art is to inspect the upper interior region of the tube plug located between the top of the tube plug and the expander member even though the expander member may hamper access to that region of the tube plug.
Probe devices for inspecting tubularly-shaped members are known. For example, an electro-mechanical eddy current probe having a rotatable sensing head for sensing the wall thickness of and locating local defects in a tube or conduit through which it is passed is disclosed in U.S. Pat. No. 4,625,165 entitled "Tube Inspection Probe With Rotating Eddy Current Coil" issued Nov. 25, 1986 to Samuel Rothstein. Although this patent may disclose an electro-mechanical eddy current probe, this patent does not appear to disclose a system capable of inspecting the upper interior region of a tube plug having an expander member disposed therein.
A probe for longitudinally traversing and circumferentially inspecting the interior of a tube is disclosed in U.S. Pat. No. 4,772,849 entitled "Rotating Probe Head For Tube Inspection". issued Sept. 20, 1988 to Joseph A. Tedder. Although this patent may disclose a probe for inspecting the interior of a tube, this patent does not appear to disclose a system capable of inspecting the upper interior region of a tube plug having an expander member disposed therein.
Thus, although the above recited patents may disclose apparatus for inspecting tubularly-shape members, these patents do not appear to disclose a system for inspecting the upper interior region of a tube plug having an expander member disposed therein.
Consequently, what is needed is a system for inspecting a tube plug having an expander member disposed therein, wherein the system is capable of inspecting the upper interior region of the tube plug between the top of the tube plug and the top of the expander member.